1. Field of the Invention
The present invention relates to a method of detoxification of exhaust gas from an internal combustion engine and an apparatus for performing the method. More particularly, the present invention relates to a catalytic system in which main toxic components of the exhaust gas mixture are liberated from their interdependent relationship.
2. Description of the Related Art
An internal combustion engine operating on the principle of externally supplied ignition primarily emits carbon monoxide (CO), nitrogen oxides (NO.sub.x) and hydrocarbons (C.sub.m H.sub.n). While carbon monoxide and the nitrogen oxides are compounds that are unequivocally defined chemically, the terms "hydrocarbons" encompasses a great number of compounds, extending from methane to compounds having complex structures, some of which are suspected carcinogens.
The technology presently in use for cleansing the exhaust gas of internal combustion engines operating on the principle of externally supplied ignition (Otto engines), using the three-way catalytic converter and the lambda sensor, is primarily based on the following chemical reactions: EQU C.sub.m H.sub.n +(m+n/4)O.sub.2 =mCO.sub.2 +n/2H.sub.2 O (1) EQU CO+1/2O.sub.2 =CO.sub.2 ( 2) EQU CO+NO=1/2N.sub.2 ( 3) EQU C.sub.m H.sub.n +2(m+n/4)NO=(m+n/4)N.sub.2 +n/2H.sub.2 O+mCO.sub.2 ( 4) EQU CO+H.sub.2 O=CO.sub.2 +H.sub.2 ( 5) EQU CH+2H.sub.2 O=CO.sub.2 +(2+n/2)H.sub.2 ( 6) EQU H.sub.2 +NO=1/2N.sub.2 +H.sub.2 O (7) EQU H.sub.2 +1/2O.sub.2 =H.sub.2 O (8)
To remove the toxic substances CO, NO.sub.x and C.sub.m H.sub.n simultaneously, the precondition is that a fuel/air ratio of approximately the stoichiometric value, lambda=1, be adhered to within narrow limits. Only within this narrow range, known as the "lambda window", is a high conversion rate for the three toxic components attained. With modified catalytic converters, a certain widening of the "lambda window" is attainable.
The precondition for high conversion rates is that the fuel/air ratio fluctuates within a narrow range about the ratio of lambda=1, which is theoretically required for complete combustion. This is attainable by means of a control loop including so-called lambda sensor as a device for measuring oxygen in the exhaust gases. This method furnishes the most favorable result if the exhaust gas cleansing and the fuel consumption are optimized simultaneously. These provisions are accordingly applicable only to internal combustion engines operating on the principle of externally supplied ignition. Taken all in all, cleansing the exhaust gas with a three-way catalytic converter is a compromise between exhaust gas, fuel consumption and engine performance. Complete exhaust gas detoxification has not been possible so far; detoxification rates of up to a maximum of 90% were the best that could be attained. On this subject, see the publication by Edgar Koberstein entitled: Katalysatoren zur Reinigung von Autoabgasen Catalytic Converters for Cleansing Automobile Exhaust Gases, in "Chemie in unserer Zeit" Chemistry in Our Times, Vol, 18, 1984, No. 2, pp. 37-45.
The problems of internal combustion engines operating on the principle of self-ignition, that is, diesel engines, are completely different from those of internal combustion engines having externally supplied ignition (Otto engines). Although diesel engine powered vehicles are less polluting than Otto engine vehicles without catalytic converters, nevertheless, when compared to Otto engine vehicles with properly regulated catalytic converters, the diesel vehicles are considerably more polluting. The exhaust gases contain primarily nitrogen oxides and soot along with a little CH and CO (from 1/10 to 1/20 as much as in Otto engines). However, with increasing power output and vehicle speed NO.sub.x emissions increase considerably, yet it is the nitrogen oxides that since 1977 have been classified as particularly dangerous. Even after elimination of the soot by means of filters, further exhaust gas cleansing is impossible in diesel engines, because the appropriate quantities of carbon monoxide or hydrocarbons are not present for reducing the nitrogen oxides.
Of quite different significance is the fact that present diesel systems are not being operated at optimal thermodynamic efficiency. With materials of higher thermal stability that have already been used, such as ceramics, the efficiency of diesel engines can be increased enormously, and a more favorable utilization of energy is possible with the corresponding significant fuel economy. However, when the efficiency of the diesel engine is increased in this way, emissions of nitrogen oxides increase drastically as well, while in contrast the formation of soot and emissions of carbon monoxide and hydrocarbons become practically negligible.